This invention relates to a vacuum switch including windmill-shaped electrodes therein.
FIG. 12 is a sectional view showing the overall structure of a vacuum switch which has a pair of contacts hermetically sealed within a highly evacuated vacuum vessel. An insulating cylinder 21 has attached to its opposite ends end plates 22a and 22b to constitute a vacuum vessel 23 of which an inner portion is highly evacuated. Opposingly disposed within the vacuum vessel 23 are a stationary electrode 1a secured to a tip of a stationary electrode rod 24a extending through one of the end plates 22a and a movable electrode 1b secured to a tip of a movable electrode rod 24b extending through the other of the end plates 22b. 
A bellows 25 is disposed across the movable electrode rode 24b and the end plate 22b. The bellows 25 allows the movable electrode rod 24b connected to an operating device (not shown) to be driven to move the movable electrode rod 24b in the axial direction. This movement of the movable electrode rod 24b causes the electrode 1a of the stationary side and the electrode 1b on the movable side to be brought into and out of electrical contact. In order to prevent metal vapor diffused from the arc generated across the electrodes 1a and 1b from depositing on the inner wall surface of the vacuum vessel 23, a shield 26 is mounted to the inner wall surface of the insulating cylinder 21 by a shield support 27.
The electrodes 1a and 1b of such vacuum switch have the same configuration, which are windmill type with grooves in the electrode. By the provision of these grooves, the electrical path in the electrode is limited to define an electrical path of a reciprocating loop-shape extending in the circumferential direction, whereby the arc is driven by a magnetic field to move along the circumference of the electrode, so that the arc is prevented from staying at one position to avoid a local melting of the electrode, thus improving the interrupting performance. Also, in order to obtain a strong magnetic drive force immediately after the arc generation, the structure has the arc-running surface and contact surface in accordance with each other.
FIGS. 13 to 16 illustrate a structure of a windmill-shaped electrode of a conventional vacuum switch tube disclosed for example in Japanese Patent Laid-Open No. 4-368734, FIGS. 13 and 15 being plan views and FIGS. 14 and 16 being side views.
In the figures, the electrode rod 24 (the stationary electrode rod 24a or the movable electrode rod 24b ) have thereon a windmill-shaped electrode 1 (the stationary side electrode 1a or the movable side electrode 1b). The windmill-shaped electrode 1 is integrally comprised of an auxiliary electrode 31 and a ring-shaped electrode 32. The auxiliary electrode 31 comprises a central portion 33 mounted to an end portion of the electrode rod 24, a plurality of arms 34 disposed to the central portion 33 in a windmill-shape manner or Buddhist cross-shape and extending in an arc from the central portion 33 toward the outer circumferential portion, and connecting portion 35 disposed at each of the tips of the plurality of the arms 34. The ring-shaped electrode 32 has an annular shape with its width substantially equal to the width of the arms 34 of the auxiliary electrode 31 and the ring-shaped electrode 32 is connected to the connecting portions 35.
In such an arrangement, when the windmill-shaped electrodes 1 (the stationary side electrode 1a and the movable side electrode 1b) are separated, an electric arc generates at the contacting surface of the ring-shaped electrode 32. When the arc generates at the point A of FIGS. 15 and 16, for example, an electric current 11 flowing through the arms 34 of the auxiliary electrode 31 generates a magnetic drive force F in the circumferential direction of the ring-shaped electrode 32, whereby the arc is driven to rotate around the outer circumference of the ring-shaped electrode 32.
Also, when the arc generates at the position which is not the connecting portions 35, such as the point E of FIGS. 15 and 16, for example, a magnetic drive force in the circumferential direction of the ring-shaped electrode 32 is also generated by an electric current 12 flowing into the ring-shaped electrode 32 from the arms 34 of the auxiliary electrode 31. Therefore, the arc is rotated along the ring-shaped electrode 32.
As has been described, in the conventional windmill-shaped electrode 1, the arc generates at the ring-shaped electrode 32 and the arc is magnetically driven immediately after the arc generation. As a result of this, the local temperature rises at the windmill-shaped electrodes 1 due to the arc before it is magnetically driven after the arc generation, thus improving the interrupting performance.
In the windmill-shaped electrode 1 of the above-described conventional vacuum switch tube, when an electric arc is generated at a point E1 of FIG. 17, for example, between the neighboring connecting portions 35 and 35, in addition to the current I2 flowing into the arc through the arms 34a, an electric current I3 from the arm 34b flows. This current I3 generates a force F3 in the direction of preventing the rotation of the arc, so that the time from the arc generation until the magnetic driving of the arc cannot be made short, not improving the interrupting performance.
Accordingly, an object of the present invention is to provide a vacuum switch free from the above-discussed problems of the conventional vacuum switch.
Another object of the present invention is to provide a vacuum switch in which an electric arc can be strongly magnetically driven immediately after the arc generation irrespective of the position on the contacting surface between the stationary side electrode and the movable side electrode at which the arc is generated, thereby improving the interrupting performance.
With the above objects in view, the present invention resides in a vacuum switch comprising: a pair of windmill-shaped electrodes disposed within a vacuum tube and each having formed therein a plurality of spiral grooves extending from a central portion to a circumferential portion thereof, and including a windmill-shaped portion separated from each other by said grooves and a plurality of contact portions separated by said grooves and having a thickness larger than that of said windmill portion; said windmill-shaped electrodes being arranged such that said contact portions are brought into contact with each other when said pair of windmill-shaped electrodes are closed, an electric arc is generated on said contact portions when said pair of windmill-shaped electrodes are separated from each other, a magnetic flux is generated by an electric current flowing into the electric arc from said windmill portion, and that a component parallel to a contact surface of said magnetic flux and serving as an arc driving force with respect to a range of 0.5 mm from the contacting surface contacting with said contacting portion of the leg portion of said arc has a magnetic flux density equal to or larger than 0.01 tesla with respect to an electric current of 1 kA.
A ratio of an inner diameter Di of said contact portion to an outer diameter D of said windmill-shaped electrodes may be equal to or greater than 0.4.
The difference in thickness between the windmill portions and the contact portions may be equal to or less than 5 mm.
Each of said windmill-shaped electrodes may be connected to each of the pair of electrode rods, a ratio of a diameter d of said connection portion of said electrode rod to an inner diameter Di of said contacting portion may be equal to or less than 0.6.
The windmill-shaped electrodes may be made of a Cuxe2x80x94Cr material including 20xe2x80x9460 weight % of Cr.